Distance Relay

Distance and Impedance relay

Brief History • 1921 Voltage restrained time over current

• 1929 balance beam impedance

• 1950 Induction cup phase comparator

• 1965 Solid state implementations

• 1984 Microprocessor based implementation

Distance Relay

Need

• Faults level are higher in high voltage transmission line

• Faults need to be cleared rapidly to avoid instability and

extensive damage

Advantages

• The impedance zone has a fix impedance rich

• Great instantaneous trip coverage with security

• Easier setting calculations and coordination

• Fixed zone of protection that are relatively impedance of the

system change

• Higher independence of load

Basic Principle

• A distance relay has the ability to detect a fault within a pre-set distance

along a transmission line or power cable from its location. Every power

line has a resistance and reactance per kilometer related to its design and

construction so its total impedance will be a function of its length. A

distance relay therefore looks at current and voltage and compares these

two quantities on the basis of Ohm’s law.

Basic Principle Ctd..

Since the impedance of a transmission line is proportional to its length, for distance measurement it is appropriate to use a relay capable of measuring the impedance of a line up to a predetermined point (the reach point). Distance relay is designed to operate only for faults occurring between the relay location and the predetermined (reach) point, thus giving discrimination for faults that may occur in different line sections. The basic principle of distance protection involves the division of voltage at the relaying point by the measured current. The calculated apparent impedance is compared with the reach point impedance. If the measured impedance is less than the reach point impedance, it is assumed that a fault exists on the line between the relay and the reach point.

Advantage of Distance Relay

• the key advantage of distance protection is that its fault

coverage of the protected circuit is virtually independent of

source impedance variations.

Transmission Line Impedance • Z ohms/mile = Ra+ j (Xa+ Xd)

• Ra, Xa function of conductor type, length Xd function of

conductor spacing, length Xa+ Xd >> Ra at higher voltages

Line Impedance

Zones of Protection • Zone 1: this is set to protect between 80% of the line length AB and

operates without any time delay. This “under-reach” setting has been

purposely chosen to avoid “over-reaching” into the next line section to

ensure selectivity since errors and transients can be present in the voltage

and current transformers. Also manufacturing tolerances limit the

measurement accuracy of the relays.

• Zone 2: this is set to protect 100% of the line length AB, plus at least

20% of the shortest adjacent line BC and operates with time delay t2.

(≈0.5s) It not only covers the remaining %20 of the line, but also provides

backup for the next line section.

• Zone 3: this is set to protect 100% of the two lines AB, BC, plus about

25% of the third line CD and operates with time delay t3. (≈1.5s)

P-Q and R-X relationships

If the direction of real and reactive

power is from bus 1 to bus 2, the R-X

coordinates are located in quadrant I. If

the direction of real power is from bus 1

to bus 2 and the reactive power is from

bus 2 to bus 1, then the R-X coordinates

are located in quadrant IV

Tripping Characteristics

The shape of the operation zones has developed throughout the years. An

overview of relay characteristics can be seen below:

Impedance Characteristic: If the relay’s operating boundary is plotted on an R-X diagram, its

impedance characteristic is a circle with its center at the origin of the

coordinates and its radius will be the setting (the reach point) in ohms.

The relay will operate for all values less than its setting i.e. for all points

within the circle. This type of relay, however, is non-directional. It can

operate for faults behind the relaying point. It takes no account of the

phase angle between voltage and current. It is also sensitive to power

swings and load encroachment due to the large impedance circle.

Mho Characteristic

The limitation of the impedance characteristic can be improved by a

technique known as self polarization. Additional voltages are fed into the

comparator in order to compare the relative phase angles of voltage and

current, so providing a directional feature. This has the effect of moving

the circle such that the circumference of the circle passes through the

origin. Angle 𝜃 is known as the relay’s characteristic angle. It appears as a

straight line on an admittance diagram.

Mho Characteristic Ctd…

By use of a further technique of feeding in voltages from the healthy phases

into the comparator (known as cross polarization) a reverse movement or

offset of the characteristic can be obtained. This is called the offset mho

characteristic.

Mho Circle Component

The mho circle is composed of its impedance maximum reach, maximum torque angle and relay characteristic angle

• Impedance Maximum Reach:

The mho circle maximum reach is set by the impedance reach Zr of the protective zone. These impedance reaches vary depending on the zone of protection such as zone 1, 2, 3 and 4. Each impedance value determines the

diameter of the mho circle.

• Maximum Torque Angle (MTA):

The angle of maximum torque of a distance relay using the mho characteristic is the angle at which it has the maximum reach. For microprocessor relays, the MTA is the same as the positive sequence line impedance angle.

• Relay Characteristic Angle:

The relay characteristic angle (RCA) of a mho circle is 90°. For purposes of calculating the maximum relay loadability, the RCA is the angle whose vertices are made between the load impedance vector and the difference between the line impedance and load impedance vectors

Mho Circle Component

Relay Impedances - Zr

•The relay impedances are the values

that a distance relay uses for zones of

protection

•Diameter of each mho circle

is based on the value of each zone of

protection

•During steady-state or normal system

conditions, the load impedance remains

constant and is high enough to

keep it away from the relay zones of

protection

Hint: As load increases, the load

apparent impedance decreases,

moving it closer to the origin

High Load

Quadrilateral Characteristic Modern distance relays offer quadrilateral characteristic, whose

resistive and reactive reach can be set independently. It

therefore provides better resistive coverage than any mho-

type characteristic for short lines. This is especially true for

earth fault impedance measurement, where the arc resistances

and fault resistance to earth contribute to the highest values

of fault resistance. Polygonal impedance characteristics are

highly flexible in terms of fault impedance coverage for both

phase and earth faults. For this reason, most digital relays

offer this form of characteristic.

Quadrilateral Characteristic

Zone Protection Calculation

Example

𝑉1 = 138∠0°𝐾𝑉 = 𝑉𝐿𝑁 = 79.6∠0°𝐾𝑉

𝑉2 = 138∠30°𝐾𝑉 = 𝑉𝐿𝑁 = 79.6∠30°𝐾𝑉

−79.6∠0°𝐾𝑉 + 𝐼66∠75° + 79.6∠30°𝐾𝑉 = 0

𝐼 =79.6∠0°𝐾𝑉-79.6∠30°𝐾𝑉

66∠75° =10.97 − 𝑗39.8

66∠75° =41.28 𝐾𝑉∠ −74.75°

66∠75° = 625.45∠ −150°

Example Ctd…

S@𝐵𝑢𝑠1 = 79.6𝐾𝑉 ∗ (−537.88 − 𝑗312.5 𝐴𝑚𝑝𝑠)∗

S@𝐵𝑢𝑠1 = 79.6𝐾𝑉 ∗ (−537.88 + 𝑗312.5 𝐴𝑚𝑝𝑠)

S@𝐵𝑢𝑠1 = −42.48𝑀𝑉𝐴 + 𝑗24.87𝑀𝑉𝐴

S@𝐵𝑢𝑠1 = 49.22∠150°

S@𝐵𝑢𝑠1 = −42.48𝑀𝑤𝑎𝑡𝑠 + 𝑗24.87𝑀𝑣𝑎𝑟𝑠

Example Ctd…

𝑍𝑙𝑜𝑎𝑑 =79.6∠0°∗

49.22∠150°∗ =79.6𝐾𝑉2∠150°

49.22𝑀𝑉𝐴= 128.73∠150°𝑂ℎ𝑚𝑠

𝑍𝑙𝑜𝑎𝑑 = −110.70 + j64.35 Ohms

Example Ctd…

The load impedance has a negative R value and a positive jX value, and those

coordinates are located in quadrant II, as indicated in Figure below. This example has

demonstrated that the location of the load impedance depends on the direction of

power. This concept will be emphasized even further when we calculate the maximum

loadability of the relay for different power factor conditions.

𝑍𝑙𝑜𝑎𝑑 = −110.70 + j64.35 Ohms

Calculating the Maximum Loadability of a Distance

Relay with mho Characteristics

• The relay impedances zones of protection must be selected

carefully in order to avoid load encroachment problems. The

zone of protection with greater risk is zone 3, since it is the

mho circle with the greatest area and closest proximity to the

load impedance. Zone 3 settings are certainly vulnerable to

load encroachment conditions during high load and power

swings conditions, which can cause the load impedance to

travel towards the boundaries of the zone 3 mho circle and

cause an undesired trip.

Example

Maximum relay loadability that zone 3 will calculated by

following steps:

1. Draw the zone 3 impedance vector in the R-X diagram.

2. Draw the load impedance vector at a specified power factor.

For this example, .

3. Draw a right triangle forming the 90 ° relay characteristic

between the load impedance vector and the difference vector

that is made up of Z3 – Zload.

Calculating the Maximum Load ability of a Distance

Relay with mho Characteristics

Example Ctd…

4. Calculate the interior angle that is made between the load and

line impedance vectors. This is done by subtracting the line

impedance angle minus the power factor angle

5. Calculate the load impedance that the relay will experience at

the specified power factor using right triangle properties:

Example Ctd…

6. Calculate the maximum loadability of the relay in MVA by:

Relay will Trip at 269 MVA